This application is related to Japanese application No. 2000-133750 filed on May 5, 2000, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a method for fabricating a mask.
2. Description of Related Art
In a conventional photolithography process for fabricating semiconductor devices, a mask obtained by forming a chrome film as a light-shielding film in a certain configuration on a quartz substrate has been generally utilized.
The patterning of the chrome film is generally performed by a lithography step of applying a resist to the quartz substrate on which the chrome film has been formed and patterning the resist using an electron beam (hereinafter referred to as EB), and an etching step of patterning the chrome film using the patterned resist as a mask.
In the etching step of the chrome film, wet etching has been customarily used and still goes mainstream in the mask fabrication. This is an opposite tendency to a wafer process in factory manufacture that has been studying and utilizing dry etching from an early stage. There are two reasons for this.
First, the miniaturization of the mask has not been required very much. In the wafer process, a reduction projection aligner (hereinafter referred to as a stepper) has realized miniaturization in the photolithography. Accordingly, the pattern of the mask has been satisfactory in the order of 5 times or 10 times larger than a pattern to be formed on the wafer. Therefore the mask miniaturization has been less required.
Second, wet etching is isotropic in general so that etching shift surely occurs. Further, if a film to be etched includes step difference or variation in film thickness, the resulting pattern configuration of the film will not be uniform. Therefore, in the wafer process, dry etching has been employed to inhibit the etching shift and the variation in the pattern configuration. On the other hand, since there is no step difference and variation in thickness on the mask substrate, it has not been necessary to deal with them in the mask fabrication.
In the advanced technology, dry etching is getting required and applied to fabricate the mask.
The first reason is that the mask pattern size of 4 times larger than a pattern to be formed on the wafer is required because a scanner system is getting commonly used in the photolithography process instead of the conventionally utilized stepper system. All commercially available scanner systems form the resist pattern on a wafer by one-fourth reduction exposure. In short, reduction ratio of a mask is 4 times and it is smaller than a stepper system.
The second reason is that as the wafer is further miniaturized, a relationship between exposure light wavelength and patterning size is reversed and needs for a proximity exposure effect correction mask is increased. In the case where an object (e.g., a conductive film, an insulating film, a resist film) is patterned on the wafer in the order smaller than the exposure light wavelength by photolithography, it is necessary to precisely adjust the amount of light transmitting from an aperture of resist (light intensity) and influence of the light diffraction. This adjustment requires extremely high resolution as compared with the prior art because a microscopic pattern that cannot be resolved on the wafer must be formed precisely on the mask. Accordingly, the miniaturization of the mask is an exigent objective.
Thus, forming the mask pattern by dry etching can improve the pattern configuration (edge roughness and sectional configuration) and the resolution of the microscopic pattern.
At present, dry etching for fabricating the mask pattern generally utilizes a mixture gas of chlorine or dichloromethane and oxygen. In this case, how to alleviate difference in etching rate depending on area to be etched is important to form a uniform and highly precise chrome mask pattern.
For example, in the case shown in FIG. 5, a resist pattern is formed on a chrome film which has been formed on a quartz substrate 40 and the chrome film is patterned into a chrome pattern 41 using the resist pattern as a mask. At this stage, the resist surrounding regions A, B and C are different in area, which varies the etching rate of the chrome film in each region. As a result, the resulting chrome patterns vary in size. In FIG. 5, there is established a relationship among the width of space between the chrome pattern lines in the region A greater than the width of space between the chrome pattern lines in the region B greater than the width of space between the chrome pattern lines in the region C. That is, a difference between the space width of the chrome pattern lines in the region A and the space width of the chrome pattern lines in the region C is 0.02 xcexcm. This is considered because molecules of the resist are decomposed through the etching and generate hydrogen ions, which inhibit the etching of the chrome film. Therefore, the more the resist exists around the chrome film to be etched, the greater the hydrogen ions occur, and as a result, the etching rate of the chrome film decreases.
Further, in the case shown in FIG. 6 in which a chrome film 51 is etched using a positive resist pattern 50 which is commonly used in the stepper or scanning projection aligner as a mask, the chrome film will have variation in width of the patterned lines in regions D, E and F due to the existence of the resist film 52 formed on the periphery of a unit cell (chip). In FIG. 6, there is established a relationship among the width of the patterned chrome line in the region D greater than the width of the line in the region E greater than the width of the patterned chrome line in the region F.
Since the difference in etching rate of the chrome film depending on the configuration of the resist pattern (resist area) impedes the formation of the precise chrome pattern, such difference must be reduced or inhibited in order to form a highly precise proximity exposure effect correction mask.
Japanese Unexamined Patent Publication No. Hei 8 (1996)-234410 proposes a method for inhibiting the difference in etching rate of the chrome film depending on the surrounding resist by providing on the chip periphery a dummy pattern for correcting the uniformity of the dry etching rate. According to this method, the size variation among the patterned chrome lines on regions E and F positioned near the chip periphery and the patterned chrome line on the region D in the chip center can be reduced. However, the variation depending on the layout of the pattern lines as shown in FIG. 5 cannot be reduced.
Further, Japanese Unexamined Patent Publication No. Hei 9 (1997)-311432 proposes a method for forming a dummy pattern having almost the same width as that of the actual pattern in a semiconductor chip. According to the method, the difference in density of the pattern lines in the semiconductor chip is alleviated and thus the size variation of the resulting patterns can be reduced. However, this method is intended for fabricating the semiconductor chips. For fabricating the mask, as shown in FIG. 6, the large resist film 52 remains in the chip periphery so that the size variation among the chrome pattern lines in the regions E and F near the periphery and the region D in the chip center cannot be reduced.
Still further, a method of dry etching the chrome film using a mixture gas prepared by adding H2 or HCl to Cl2 gas has been proposed (Photomask Japan ""99 Proceeding 137p). According to the method, the concentration of the hydrogen ions inhibiting the chrome etching is controlled to be uniform on the mask surface by supplying the hydrogen ions to the etching gas. Accordingly the etching rate can be uniform on the entire surface, though the etching rate of the chrome film is lowered in total. Thus, the size variation in chrome pattern lines can be reduced.
However, as shown in FIG. 7, the chrome pattern lines still vary in width even though the optimized etching gas is used. The reason is considered that the positive resist pattern used to pattern the chrome film has not been uniformly formed. That is, in the lithography step using EB, a phenomenon called fogging occurs, in which electrons that once entered the resist film are reflected out, and reflected again on the EB optical system and then re-enter the resist film. Therefore, EB dosage is not uniformly formed depending on the configuration of the resist pattern to be obtained, and as a result, the variation of the resist pattern occurs. For example, in the case of forming a resist pattern corresponding to the chrome pattern shown in FIG. 5, the amount of electrons that re-enters the resist film due to the fogging increases in the region A surrounded by the large exposure area. Accordingly, the space width between the patterned resist lines increases as compared with that in the region B.
The fogging theoretically tends to be amplified depending on the acceleration voltage and the light exposure amount of the EB exposure system. However, it is impossible so far to completely inhibit the phenomenon by the currently available systems.
Further, the size variation among the resist pattern lines caused by the fogging proceeds in the same direction as the variation of the chrome patterns caused by the etching. Therefore the reduction of the variation is quite difficult.
The present invention has been achieved in view of the above. The object of the present invention is to provide a method for fabricating a mask capable of inhibiting the difference in the etching rate of the chrome film due to the fogging and the area of the surrounding resist, regardless of the pattern layout in LSI.
Accordingly, the present invention provides a method for fabricating a mask comprising the steps of: fabricating a light-shielding film on an entire surface of a substrate including an actual pattern region and an unoccupied region other than the actual pattern region; patterning the light-shielding film on the actual pattern region while leaving the light-shielding film on the unoccupied region; and removing the light-shielding film on the unoccupied region while leaving the patterned light-shielding film on the actual pattern region.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.